Your T is a vector, and your Y is a scalar. There is no singular line because it is actually plotting all 5 points as individual line objects. check it out:
The above code should plot 5 circular markers.
a will return 5 line objects.
if you ran this:
then a will return 1 line object, that is a line.
also, dont put a space between plot and the parentheses.
let me know if this clears your question